[ICE]: parallel: `variances.get(i).unwrap()` None

[matthiaskrgr]

Code

pub struct T<'a>(&'a str);

pub fn f<T>() -> _ {
    T
}

pub fn g<'a>(val: T<'a>) -> _ {
    T
}

fn main() {}

Meta

rustc --version --verbose:

rustc 1.96.0-nightly (fb27476aa 2026-03-28)
binary: rustc
commit-hash: fb27476aaf1012f1f6ace6306f9b990e0d989c31
commit-date: 2026-03-28
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.2

[matthiaskrgr]

sometimes, the output is

error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
 --> a.rs:7:29
  |
7 | pub fn g<'a>(val: T<'a>) -> _ {
  |                             ^ not allowed in type signatures
  |
help: replace with the correct return type
  |
7 - pub fn g<'a>(val: T<'a>) -> _ {
7 + pub fn g<'a>(val: T<'a>) -> fn(&str) -> T<'_> {
  |

error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
 --> a.rs:3:18
  |
3 | pub fn f<T>() -> _ {
  |                  ^ not allowed in type signatures
  |
help: replace with the correct return type
  |
3 - pub fn f<T>() -> _ {
3 + pub fn f<T>() -> fn(&'static str) -> T<'static> {
  |

warning: unused variable: `val`
 --> a.rs:7:14
  |
7 | pub fn g<'a>(val: T<'a>) -> _ {
  |              ^^^ help: if this is intentional, prefix it with an underscore: `_val`
  |
  = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

error: aborting due to 2 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0121`.

sometimes the output is

error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
 --> a.rs:3:18
  |
3 | pub fn f<T>() -> _ {
  |                  ^ not allowed in type signatures
  |
help: replace with the correct return type
  |
3 - pub fn f<T>() -> _ {
3 + pub fn f<T>() -> fn(&'static str) -> T<'static> {
  |

error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
 --> a.rs:7:29
  |
7 | pub fn g<'a>(val: T<'a>) -> _ {
  |                             ^ not allowed in type signatures
  |
help: replace with the correct return type
  |
7 - pub fn g<'a>(val: T<'a>) -> _ {
7 + pub fn g<'a>(val: T<'a>) -> fn(&str) -> T<'_> {
  |

warning: unused variable: `val`
 --> a.rs:7:14
  |
7 | pub fn g<'a>(val: T<'a>) -> _ {
  |              ^^^ help: if this is intentional, prefix it with an underscore: `_val`
  |
  = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

error: aborting due to 2 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0121`.

sometimes it is

error[E0391]: cycle detected when computing function signature of `f`
 --> a.rs:3:1
  |
3 | pub fn f<T>() -> _ {
  | ^^^^^^^^^^^^^^^^^^
  |
note: ...which requires type-checking `f`...
 --> a.rs:3:1
  |
3 | pub fn f<T>() -> _ {
  | ^^^^^^^^^^^^^^^^^^
note: ...which requires computing the variances of `T`...
 --> a.rs:1:1
  |
1 | pub struct T<'a>(&'a str);
  | ^^^^^^^^^^^^^^^^
  = note: ...which requires computing the variances for items in this crate...
  = note: ...which again requires computing function signature of `f`, completing the cycle
note: cycle used when checking that `f` is well-formed
 --> a.rs:3:1
  |
3 | pub fn f<T>() -> _ {
  | ^^^^^^^^^^^^^^^^^^
  = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0391`.

and sometimes it is

thread 'rustc' (1096231) panicked at /rustc-dev/fb27476aaf1012f1f6ace6306f9b990e0d989c31/compiler/rustc_type_ir/src/relate.rs:142:41:
called `Option::unwrap()` on a `None` value
stack backtrace:
   0:     0x7f64da3845ab - <<std[78f1e358c25996ef]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[cd88f6cf038557fb]::fmt::Display>::fmt
   1:     0x7f64daa2f608 - core[cd88f6cf038557fb]::fmt::write
   2:     0x7f64da39b5b6 - <std[78f1e358c25996ef]::sys::stdio::unix::Stderr as std[78f1e358c25996ef]::io::Write>::write_fmt
   3:     0x7f64da35a5b8 - std[78f1e358c25996ef]::panicking::default_hook::{closure#0}
   4:     0x7f64da3779b3 - std[78f1e358c25996ef]::panicking::default_hook
   5:     0x7f64d937cb8c - std[78f1e358c25996ef]::panicking::update_hook::<alloc[5b34cacabfc0eb1b]::boxed::Box<rustc_driver_impl[e37ea5797c2c1a39]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f64da377c92 - std[78f1e358c25996ef]::panicking::panic_with_hook
   7:     0x7f64da35a6aa - std[78f1e358c25996ef]::panicking::panic_handler::{closure#0}
   8:     0x7f64da3513a9 - std[78f1e358c25996ef]::sys::backtrace::__rust_end_short_backtrace::<std[78f1e358c25996ef]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f64da35c0dd - __rustc[b27ca4e69fe37a8]::rust_begin_unwind
  10:     0x7f64d74e6a2c - core[cd88f6cf038557fb]::panicking::panic_fmt
  11:     0x7f64d6a06bb2 - core[cd88f6cf038557fb]::panicking::panic
  12:     0x7f64d711da19 - core[cd88f6cf038557fb]::option::unwrap_failed
  13:     0x7f64db6658e0 - rustc_type_ir[fd280ca1c2e42b66]::relate::relate_args_with_variances::<rustc_middle[98a1695a658fa77e]::ty::context::TyCtxt, rustc_infer[8d84b0296f7e2798]::infer::relate::generalize::Generalizer>
  14:     0x7f64daa88527 - rustc_type_ir[fd280ca1c2e42b66]::relate::structurally_relate_tys::<rustc_middle[98a1695a658fa77e]::ty::context::TyCtxt, rustc_infer[8d84b0296f7e2798]::infer::relate::generalize::Generalizer>::{closure#0}
  15:     0x7f64daa81e3d - <rustc_infer[8d84b0296f7e2798]::infer::relate::generalize::Generalizer as rustc_type_ir[fd280ca1c2e42b66]::relate::TypeRelation<rustc_middle[98a1695a658fa77e]::ty::context::TyCtxt>>::tys::{closure#0}
  16:     0x7f64daa6dbc5 - <rustc_infer[8d84b0296f7e2798]::infer::relate::type_relating::TypeRelating as rustc_type_ir[fd280ca1c2e42b66]::relate::TypeRelation<rustc_middle[98a1695a658fa77e]::ty::context::TyCtxt>>::tys
  17:     0x7f64db6bf8d6 - <rustc_hir_typeck[9e63f00e0269f555]::coercion::Coerce>::unify_raw
  18:     0x7f64dabe2a43 - <rustc_hir_typeck[9e63f00e0269f555]::fn_ctxt::FnCtxt>::coerce
  19:     0x7f64daba6fd4 - <rustc_hir_typeck[9e63f00e0269f555]::fn_ctxt::FnCtxt>::check_expr_block
  20:     0x7f64dabb5e95 - <rustc_hir_typeck[9e63f00e0269f555]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  21:     0x7f64db07da28 - rustc_hir_typeck[9e63f00e0269f555]::check::check_fn
  22:     0x7f64db0aeea5 - rustc_hir_typeck[9e63f00e0269f555]::typeck_with_inspect::{closure#0}
  23:     0x7f64db0ac0c4 - rustc_query_impl[86d49cf1f0d19028]::query_impl::typeck::invoke_provider_fn::__rust_begin_short_backtrace
  24:     0x7f64daae16a6 - rustc_query_impl[86d49cf1f0d19028]::execution::try_execute_query::<rustc_data_structures[c577e586c38a8d20]::vec_cache::VecCache<rustc_span[d062e5c8b9463d68]::def_id::LocalDefId, rustc_middle[98a1695a658fa77e]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[98a1695a658fa77e]::dep_graph::graph::DepNodeIndex>, false>
  25:     0x7f64daae1043 - rustc_query_impl[86d49cf1f0d19028]::query_impl::typeck::execute_query_non_incr::__rust_end_short_backtrace
  26:     0x7f64dafa1f0d - rustc_hir_analysis[872333e2b5cbbb56]::collect::lower_fn_sig_recovering_infer_ret_ty
  27:     0x7f64dad76618 - rustc_hir_analysis[872333e2b5cbbb56]::collect::fn_sig
  28:     0x7f64dad75ce6 - rustc_query_impl[86d49cf1f0d19028]::query_impl::fn_sig::invoke_provider_fn::__rust_begin_short_backtrace
  29:     0x7f64dad7a246 - rustc_query_impl[86d49cf1f0d19028]::execution::try_execute_query::<rustc_middle[98a1695a658fa77e]::query::caches::DefIdCache<rustc_middle[98a1695a658fa77e]::query::erase::ErasedData<[u8; 24usize]>>, false>
  30:     0x7f64dad79825 - rustc_query_impl[86d49cf1f0d19028]::query_impl::fn_sig::execute_query_non_incr::__rust_end_short_backtrace
  31:     0x7f64db7644c0 - rustc_hir_analysis[872333e2b5cbbb56]::check::check::check_item_type
  32:     0x7f64db75f426 - rustc_hir_analysis[872333e2b5cbbb56]::check::wfcheck::check_well_formed
  33:     0x7f64db75f407 - rustc_query_impl[86d49cf1f0d19028]::query_impl::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
  34:     0x7f64db75ea24 - rustc_query_impl[86d49cf1f0d19028]::execution::try_execute_query::<rustc_data_structures[c577e586c38a8d20]::vec_cache::VecCache<rustc_span[d062e5c8b9463d68]::def_id::LocalDefId, rustc_middle[98a1695a658fa77e]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[98a1695a658fa77e]::dep_graph::graph::DepNodeIndex>, false>
  35:     0x7f64db75e7bb - rustc_query_impl[86d49cf1f0d19028]::query_impl::check_well_formed::execute_query_non_incr::__rust_end_short_backtrace
  36:     0x7f64d94e70ba - <rustc_middle[98a1695a658fa77e]::hir::ModuleItems>::par_opaques::<rustc_hir_analysis[872333e2b5cbbb56]::check::wfcheck::check_type_wf::{closure#5}>::{closure#0}
  37:     0x7f64d957b2bb - <rustc_thread_pool[20b626c379f01edb]::job::HeapJob<<rustc_thread_pool[20b626c379f01edb]::scope::Scope>::spawn<rustc_data_structures[c577e586c38a8d20]::sync::parallel::par_slice<&rustc_hir[c1216199efaeeb18]::hir::ImplItemId, rustc_data_structures[c577e586c38a8d20]::sync::parallel::try_par_for_each_in<&[rustc_hir[c1216199efaeeb18]::hir::ImplItemId], rustc_span[d062e5c8b9463d68]::ErrorGuaranteed, <rustc_middle[98a1695a658fa77e]::hir::ModuleItems>::par_impl_items<rustc_hir_analysis[872333e2b5cbbb56]::check::wfcheck::check_type_wf::{closure#1}>::{closure#0}>::{closure#0}::{closure#0}>::{closure#1}::{closure#0}>::{closure#0}> as rustc_thread_pool[20b626c379f01edb]::job::Job>::execute
  38:     0x7f64da06ec3f - <rustc_thread_pool[20b626c379f01edb]::registry::WorkerThread>::wait_or_steal_until_cold
  39:     0x7f64da06d7d2 - <rustc_thread_pool[20b626c379f01edb]::registry::ThreadBuilder>::run
  40:     0x7f64d9375cbc - std[78f1e358c25996ef]::sys::backtrace::__rust_begin_short_backtrace::<<rustc_thread_pool[20b626c379f01edb]::ThreadPoolBuilder>::build_scoped<rustc_interface[89f5b40f13318e16]::util::run_in_thread_pool_with_globals<rustc_interface[89f5b40f13318e16]::interface::run_compiler<(), rustc_driver_impl[e37ea5797c2c1a39]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#0}, rustc_interface[89f5b40f13318e16]::util::run_in_thread_pool_with_globals<rustc_interface[89f5b40f13318e16]::interface::run_compiler<(), rustc_driver_impl[e37ea5797c2c1a39]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#1}, ()>::{closure#0}::{closure#0}::{closure#0}, ()>
  41:     0x7f64d937f7d3 - <std[78f1e358c25996ef]::thread::lifecycle::spawn_unchecked<<rustc_thread_pool[20b626c379f01edb]::ThreadPoolBuilder>::build_scoped<rustc_interface[89f5b40f13318e16]::util::run_in_thread_pool_with_globals<rustc_interface[89f5b40f13318e16]::interface::run_compiler<(), rustc_driver_impl[e37ea5797c2c1a39]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#0}, rustc_interface[89f5b40f13318e16]::util::run_in_thread_pool_with_globals<rustc_interface[89f5b40f13318e16]::interface::run_compiler<(), rustc_driver_impl[e37ea5797c2c1a39]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#1}, ()>::{closure#0}::{closure#0}::{closure#0}, ()>::{closure#1} as core[cd88f6cf038557fb]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  42:     0x7f64dbbcd8ec - <std[78f1e358c25996ef]::sys::thread::unix::Thread>::new::thread_start
  43:     0x7f64d56a597a - <unknown>
  44:     0x7f64d57292bc - <unknown>
  45:                0x0 - <unknown>

error: the compiler unexpectedly panicked. This is a bug

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/tmp/im2/rustc-ice-2026-03-29T19_55_22-1096218.txt` to your bug report

note: rustc 1.96.0-nightly (fb27476aa 2026-03-28) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z threads=20 --crate-type lib

query stack during panic:
#0 [typeck] type-checking `f`
#1 [fn_sig] computing function signature of `f`
#2 [check_well_formed] checking that `f` is well-formed
#3 [check_type_wf] checking that types are well-formed
#4 [analysis] running analysis passes on crate `a`
end of query stack
note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error[E0391]: cycle detected when computing the variances for items in this crate
  |
note: ...which requires computing function signature of `f`...
 --> a.rs:3:1
  |
3 | pub fn f<T>() -> _ {
  | ^^^^^^^^^^^^^^^^^^
note: ...which requires type-checking `f`...
 --> a.rs:3:1
  |
3 | pub fn f<T>() -> _ {
  | ^^^^^^^^^^^^^^^^^^
note: ...which requires computing the variances of `T`...
 --> a.rs:1:1
  |
1 | pub struct T<'a>(&'a str);
  | ^^^^^^^^^^^^^^^^
  = note: ...which again requires computing the variances for items in this crate, completing the cycle
note: cycle used when computing the variances of `T`
 --> a.rs:1:1
  |
1 | pub struct T<'a>(&'a str);
  | ^^^^^^^^^^^^^^^^
  = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
  = note: delayed at /rustc-dev/fb27476aaf1012f1f6ace6306f9b990e0d989c31/compiler/rustc_query_impl/src/job.rs:506:20
             0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
             1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
             2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
             3: <rustc_errors::diagnostic::Diag>::delay_as_bug
             4: <rustc_query_impl::handle_cycle_error::specialize_query_vtables::{closure#3} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_span::def_id::DefId, rustc_middle::query::plumbing::Cycle, rustc_errors::diagnostic::Diag)>>::call_once
             5: rustc_query_impl::execution::handle_cycle::<rustc_middle::query::caches::DefIdCache<rustc_middle::query::erase::ErasedData<[u8; 16]>>>
             6: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::DefIdCache<rustc_middle::query::erase::ErasedData<[u8; 16]>>, false>
             7: rustc_query_impl::query_impl::variances_of::execute_query_non_incr::__rust_end_short_backtrace
             8: rustc_type_ir::relate::structurally_relate_tys::<rustc_middle::ty::context::TyCtxt, rustc_infer::infer::relate::generalize::Generalizer>::{closure#0}
             9: <rustc_infer::infer::relate::generalize::Generalizer as rustc_type_ir::relate::TypeRelation<rustc_middle::ty::context::TyCtxt>>::tys::{closure#0}
            10: <rustc_infer::infer::relate::type_relating::TypeRelating as rustc_type_ir::relate::TypeRelation<rustc_middle::ty::context::TyCtxt>>::tys
            11: <rustc_hir_typeck::coercion::Coerce>::unify_raw
            12: <rustc_hir_typeck::fn_ctxt::FnCtxt>::coerce
            13: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_block
            14: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
            15: rustc_hir_typeck::check::check_fn
            16: rustc_hir_typeck::typeck_with_inspect::{closure#0}
            17: rustc_query_impl::query_impl::typeck::invoke_provider_fn::__rust_begin_short_backtrace
            18: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 8]>, rustc_middle::dep_graph::graph::DepNodeIndex>, false>
            19: rustc_query_impl::query_impl::typeck::execute_query_non_incr::__rust_end_short_backtrace
            20: rustc_hir_analysis::collect::lower_fn_sig_recovering_infer_ret_ty
            21: rustc_hir_analysis::collect::fn_sig
            22: rustc_query_impl::query_impl::fn_sig::invoke_provider_fn::__rust_begin_short_backtrace
            23: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::DefIdCache<rustc_middle::query::erase::ErasedData<[u8; 24]>>, false>
            24: rustc_query_impl::query_impl::fn_sig::execute_query_non_incr::__rust_end_short_backtrace
            25: rustc_hir_analysis::check::check::check_item_type
            26: rustc_hir_analysis::check::wfcheck::check_well_formed
            27: rustc_query_impl::query_impl::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
            28: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 1]>, rustc_middle::dep_graph::graph::DepNodeIndex>, false>
            29: rustc_query_impl::query_impl::check_well_formed::execute_query_non_incr::__rust_end_short_backtrace
            30: <rustc_middle::hir::ModuleItems>::par_opaques::<rustc_hir_analysis::check::wfcheck::check_type_wf::{closure#5}>::{closure#0}
            31: <rustc_thread_pool::job::HeapJob<<rustc_thread_pool::scope::Scope>::spawn<rustc_data_structures::sync::parallel::par_slice<&rustc_hir::hir::ImplItemId, rustc_data_structures::sync::parallel::try_par_for_each_in<&[rustc_hir::hir::ImplItemId], rustc_span::ErrorGuaranteed, <rustc_middle::hir::ModuleItems>::par_impl_items<rustc_hir_analysis::check::wfcheck::check_type_wf::{closure#1}>::{closure#0}>::{closure#0}::{closure#0}>::{closure#1}::{closure#0}>::{closure#0}> as rustc_thread_pool::job::Job>::execute
            32: <rustc_thread_pool::registry::WorkerThread>::wait_or_steal_until_cold
            33: <rustc_thread_pool::registry::ThreadBuilder>::run
            34: std::sys::backtrace::__rust_begin_short_backtrace::<<rustc_thread_pool::ThreadPoolBuilder>::build_scoped<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#0}, rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#1}, ()>::{closure#0}::{closure#0}::{closure#0}, ()>
            35: <std::thread::lifecycle::spawn_unchecked<<rustc_thread_pool::ThreadPoolBuilder>::build_scoped<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#0}, rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#1}, ()>::{closure#0}::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
            36: <std::sys::thread::unix::Thread>::new::thread_start
            37: <unknown>
            38: <unknown>


note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/tmp/im2/rustc-ice-2026-03-29T19_55_22-1096218.txt` to your bug report

note: rustc 1.96.0-nightly (fb27476aa 2026-03-28) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z threads=20 --crate-type lib

query stack during panic:
end of query stack

[Bryntet]

I'd like to give this a try, @rustbot claim

[Bryntet]

The ICE occurs when a check_type_wf query, calls into a typeck_root query, which in turn emits an error, while separate parallel check_type_wf query is running in the background.

making check_type_wf single-threaded solves this issue, but with obvious perf regression (not sure how big though), best/easiest fix is probably just to delay_bug if variances.get(i) is None since from what I can tell this behaviour can only happen when there will already be an error that will be emitted

[zetanumbers]

How's progress on this?

[Bryntet]

Been busy with other stuff @rustbot unclaim

Hopefully my comment can be useful for someone else to give it a shot

[petrochenkov]

Triage:
This case was not added to the parallel test suite, we need to check whether the ICE still reproduces, and add this case to ui tests even if //@ ignore-parallel-frontend is required.
The non-ICE parts are fine, query cycle diagnostics are known to be non-reproducible with parallel frontend and there are other issues for that).